: The long-range goal of this project is to understand B cell tolerance and how tolerance is broken in disease. The focus is on B cells specific for the Smith (Sm) antigen, a ribonucleoprotein involved in RNA splicing found in all cells, and a target of the immune system in systemic lupus erythematosus. To understand the regulation of anti-Sm B cells Ig H chain transgenic (2-12H Tg) mice using the rearrangement from an anti-Sm B cell have been generated. Anti-Sm B cells are present in the spleens of non-autoimmune 2-12H Tg mice and most are transitional B cells. In addition, about30 percent of peritoneal B-l cells are anti-Sm in these mice. Although these mice respond to Sm immunization, indicating that tolerance is maintained by ignorance, pre-immune serum anti-Sm levels are no different from those of non-Tg mice. This is paradoxical, since B-l cells are responsible for most circulating IgM. In autoimmune MRL/lpr mice, anti-Sm B cells are activated but they do not differentiate to B-l. The three aims of this application test the following hypothesis: anti-Sm transitional B cells are driven by antigen to differentiate to B-1, but they are inhibited from activation by the expression of CD5, a negative regulator of B cell activation. In autoimmune mice, where differentiation to B-1 is blocked, anti-Sm B cells differentiate to B-2 where they are more readily activated. In the first aim, the ability of anti-Sm transitional, B cells to differentiate to B-1 will be determined by transfer of 2-12H Tg transitional. B cells to non-Tg mice. In addition, the V repertoires of anti-Sm transitional and B-1 cells of 212H Tg mice will be analyzed to determine whether differentiation to B-l is selective. In the second aim, anti-Sm B-1 cell function will be assessed in vitro and in vivo, and the role of CD5 determined in 2-12H Tg/CD54-/-mice. In the final aim, the fate of B cells that normally differentiate to B-l will be determined in autoimmune MRL/lpr mice using a Tg model system of B-l differentiation.